As Britain braces for Brexit vote, EU warns 'out is out'; opinion polls put 'Leave' and 'Remain' camps neck and neck

London: On the eve of a crucial referendum, Britain was categorically told that if it decides to leave the European Union (EU) on Thursday, there would be no coming back.

The warning by EU leaders came ahead of the Brexit vote even as two opinion polls put the 'Leave' campaign ahead of the 'Remain', though the margin was knife-edge.

While the Opinium poll put the "Leave" camp at 45 percent and "Remain" at 44 percent, TNS gave them a lead of 43 percent to 41 percent for staying.

"Our latest poll suggest that Leave is in a stronger position than Remain," Luke Taylor, head of social and political attitudes at TNS, said in a statement.

However, he cautioned that "a late swing to the status quo" was possible in the final hours before voting.

If Britons do vote in favour of Brexit, Britain would become the first nation to exit the European Union bloc in its 60-year history.

While the rest of the bloc is visibly worried at the prospect of Britain leaving the Union, European Commission chief Jean-Claude Juncker was less forgiving in his remarks.

"Out is out," Juncker said in Brussels, dismissing any talk of a post-vote renegotiation just hours before polls open.

French President Francois Hollande too warned that an exit would be "irreversible".

German Chancellor Angela Merkel, on the other hand, said it was up to Britons to decide whether they wanted to stay or not.

Merkel and Hollande are due to meet in Berlin next week for talks which the French president said would work "towards relaunching the European project", already struggling with an unprecedented migrant crisis.

British Prime Minister David Cameron, meanwhile, spent the final day of campaigning travelling around Britain on his battle bus and doing interviews.

"If I had to sum up this whole campaign in a word, it would be that word `together`," the Conservative leader, who is facing calls to resign if Brexit happens, told BBC radio.

Out on the campaign trail, he added: "If we want a bigger economy and more jobs, we are better if we do it together"

Bosses from nearly 1,300 of Britain`s leading businesses signed a letter in The Times saying the country was stronger in the EU.

James Bond star Daniel Craig and Irish rock band U2 became the latest celebrities to back

"Remain".

Despite the polls showing the race is neck and neck, bookmaker Betfair said their latest odds implied a 76-percent chance of "Remain" winning.

On the eve of the vote, planes with banners from the rival campaigns criss-crossed the skies above central London trying to woo undecided voters.

Cameron`s main rival in the "Leave" campaign and possible successor, Boris Johnson, said Britain stood on the brink of "independence day" from Europe.

"I do think that we are on the verge, possibly, of an extraordinary event in the history of our country and indeed in the whole of Europe," Johnson said in eastern England.

Nigel Farage, leader of the anti-EU UK Independence Party, said: "I genuinely believe we are going to win this."

US Republican White House hopeful Donald Trump, who arrives in Britain Thursday, also spoke out on Brexit again, saying he thought the country should "go it alone".

A British withdrawal would trigger a lengthy exit negotiation, leading to the loss of unfettered access to its partners in the 28-nation market and forcing the country to strike its own trade accords across the world.

In Europe, there are already concerns that a Brexit would imperil the integrity of the bloc, already buffeted by the eurozone and migration crises.

The "Leave" campaign briefly took a slight lead in many opinion polls until last week, sending sterling plummeting.

This fell away after campaigning was paused for two days following Thursday`s killing of pro-EU lawmaker Jo Cox of the main opposition Labour party.

Cox`s widower Brendan has said his wife, who was particularly noted for her work on refugee rights, was killed because of her political views.

(With AFP inputs)